Recursion with if statements Java

Question

Got two int a and b, need to find the sum of all the numbers between including them too.

Got Recursion Method: with two if statements, if use only one if statement code work fine, otherwise fall with StackOverflowError.

public static int getSum(int a, int b) {
    int result = 0;

    if (a <= 0 && b <= 0)  result = getSum(a + 1, b + 1) + a + b;

    if (a >= 0 && b >= 0)  result = getSum(a - 1, b - 1) + a + b;

    return  result;
}

How can I fix it?

For the record: you might want to add one example that shows a failure. Here the problem is kinda obvious, but when writing up questions, they should always represent a true minimal reproducible example. — GhostCat
– GhostCat, Commented Sep 10, 2018 at 14:05
Just in case: you just reached upvote level, so you are able to express appreciation for helpful answers beyond accepting a single one ;-) — GhostCat
– GhostCat, Commented Sep 10, 2018 at 14:33

GhostCat · Accepted Answer · 2018-09-10 13:51:36Z

Lets assume a is 1, and b is 2.

if (a <= 0 && b <= 0)  result = getSum(a + 1, b + 1) + a + b;
if (a >= 0 && b >= 0)  result = getSum(a - 1, b - 1) + a + b;

The 2nd kicks in:

result = getSum(1 - 1, 2 - 1) + a + b;

So you call: with a = 0, b = 2. That one picks:

result = getSum(0 + 1, 1 + 1) + a + b;

So you are back to call with 1, 2.

And it starts from scratch. Leading into an endless recursion.

So, there are multiple problems with your current approach:

both if conditions might apply. As you use "<=" and ">=", when a or b are 0, both if conditions kick in
worse: as shown, your setup allows to easily go down-up-down-up-... forever
beyond that: a correct "stop recursion" is missing. For certain input, your code will simply always go and call itself, again.
guessing here: you are also missing corner cases, such as a < 0, but b > 0

So, long story short: your whole algorithm is bogus, and you need to step back and rethink what you are doing here!

SomeDude · Accepted Answer · 2018-09-10 14:20:45Z

2

This doesn't need recursion, but I assume you are trying to learn recursion. Please see comments for explanation.

public static int getSum( int a, int b ) { 
 if ( a == b ) { // if both are equal there are no numbers in between - so sum is number itself.
     return a;
 }
 // if a < b then increment a to reach b otherwise increment b to reach a.
 // this works even if a or b or both are negative.
 // So when you increment a, add 'a' only to current sum and then move forward
 // When you increment b, add 'b' only to current sum and then move forward.
 return a < b ?  a + getSum( a + 1, b ) :  b + getSum( a, b + 1 );    
}

edited Sep 10, 2018 at 14:20

answered Sep 10, 2018 at 13:59

SomeDude

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1 Comment

IMBABOT Over a year ago

getSum(0, 1) should return 1, but return value 2.

Hongyu Wang · Accepted Answer · 2018-09-10 14:06:17Z

1

You don't need those if statements. Just do it as follows:

public static int getSum(int a, int b){

    if (b < a) return 0;
    if (b == a) return b;
    return a + getSum(a + 1, b);
}

edited Sep 10, 2018 at 14:06

answered Sep 10, 2018 at 13:50

Hongyu Wang

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1 Comment

TDG Over a year ago

For `a=1, b=2' the result is 1 instead of 3.

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